fix(examples): replace deprecated datetime.utcnow() in web_search_gpt_oss_helper

[Ghraven]

What

datetime.utcnow() is deprecated since Python 3.12 and emits a DeprecationWarning whenever the helper is imported under a strict warnings config. The recommended replacement is datetime.now(timezone.utc), which also returns a timezone-aware UTC datetime instead of the naive one utcnow() produces — a small correctness win, since naive UTC datetimes can silently misbehave when compared against aware ones elsewhere in user code.

Why

• Removes Python 3.12+ DeprecationWarning from the example.
• Returns aware UTC datetimes, which is the modern Python recommendation.
• Behaviour-equivalent for the example's downstream use: fetched_at: datetime accepts both naive and aware values.

Changes

examples/web_search_gpt_oss_helper.py:

• Add timezone to the existing from datetime import datetime import.
• Replace 5 occurrences of datetime.utcnow() with datetime.now(timezone.utc).

Total: +6/-6 lines, 1 file.

Testing

Pure call-site swap with no logic change. The file parses cleanly (python -m py_compile). The example still produces the same wall-clock instant for fetched_at, just with a tzinfo=UTC attached instead of None.